The present invention relates to a plugged honeycomb structure used as a filter for filtrating particulate matter, such as a diesel particulate filter, or as a catalyst carrier for purifying harmful components such as HC, NOx, and CO in exhaust gas by carrying a catalyst component on a surface of pores of porous partition walls defining cells and passing exhaust gas through the pores of the partition walls.
Since, in the exhaust gas exhausted from an internal combustion engine of a diesel engine or the like, a large amount of particulate matter having carbon which causes environmental pollution as the main component is contained, it is general that a filter for trapping (filtrating) particulate matter is mounted in an exhaust gas system thereof.
In a filter used for such a purpose, as shown in FIGS. 12(a) and 12(b), there is generally used a plugged honeycomb structure 1 having a honeycomb structure 2 where a plurality of cells 9 extending between two end faces are defined by porous partition walls 7 and plugged portions 11 disposed so as to plug one of the two opening ends of each of the cells 9 and to form a checkerwise pattern on each of the end faces complementarily between one end face and the other end face of the honeycomb structure 2.
Exhaust gas flows into the inside from one end face 3 of a filter constituted by such a plugged honeycomb structure 1, and after particulate matter and the like contained in the gas are removed, the gas flows out from the other end face 5. Specifically, exhaust gas flows into the cells 9b whose end portions are not plugged on one end face 3 and are plugged on the other end face 5 in the first place, passes through the porous partition walls 7, moves into the cells 9a whose end portions are plugged on the end face 3 and are not plugged on the other end face 5, and is discharged from the cells 9a. At this time, the partition walls 7 function as a filtration layer, and particulate matter in the gas is trapped by the partition walls 7 and accumulates on the partition walls 7.
As a method for manufacturing a plugged honeycomb structure as described above, there is known a method where an adhesive sheet or the like is bonded on one end face of a formed honeycomb body, holes are made in portions corresponding to cells to be plugged (plugged cells) of the adhesive sheet or the like by laser processing using image processing to prepare a mask, the end face having the mask bonded thereon of the formed honeycomb body is immersed in slurry for plugging to fill the slurry into the cells to be plugged of the formed honeycomb body to form plugged portions, the same process is performed with respect to the other end face of the formed honeycomb body, followed by drying and firing to obtain a plugged honeycomb structure (see, e.g., Patent Document 1).
As another manufacturing method, there has been proposed a method where through-holes are formed in an undried formed sheet-shaped green body of a ceramic material in a checkerwise pattern, and the formed sheet-shaped green body is attached to an end face of a honeycomb-shaped dried body under pressure, followed by integrally firing to obtain a plugged honeycomb structure (see, e.g., Patent Document 2).
By the way, recently, an attempt has been made to utilize such a plugged honeycomb structure as a catalyst for purifying harmful components such as HC, NOx, and CO in exhaust gas by carrying a catalyst component on a surface of pores in the porous partition walls defining the cells and passing exhaust gas flowing into the honeycomb structure through the pores of the partition walls (such a catalyst is hereinbelow referred to as a “wall-flow type catalyst”) other than a filter for trapping particulate matter as described above.
According to expansion of usage of such a plugged honeycomb structure, it is expected that a demand for a plugged honeycomb structure will increase much more in the future. In the case of trying to mass-produce plugged honeycomb structures by employing the manufacturing method. described in the Patent Document 1, it is required to solve the problems of (i) requiring steps of bonding an adhesive sheet, image processing, making holes, and peeling the adhesive sheet with respect to each of both the end faces of each honeycomb structure, which generates a large amount of industrial waste of peeled adhesive sheets after being used, in addition to requirement of much time to be spent on production of a mask, etc., and (ii) causing rise in production cost because an adhesive sheet is relatively expensive and requiring to perform bonding operation in clean circumstances since the sheet hates dirt and dust, which restricts operation circumstances. That is, though the above-mentioned conventional manufacturing method is effective in manufacturing a relatively small number of plugged honeycomb structures, a sufficient satisfaction cannot be derived from the conventional manufacturing method from the viewpoint of mass-production adaptability.
In addition, the manufacturing method described in Patent Document 2 has problems of easily having a crack in the sheet or peeling of the sheet due to a difference in drying shrinkage between a honeycomb-shaped dried body and an undried formed sheet-shaped green body and hard plugging of the cells to be plugged among the cells located in the outermost periphery of the honeycomb structure.                [Patent Document 1] JP-A-2001-300922        [Patent Document 2] JP-A-2002-159811        